The deployment of 802.11 Wireless Local Area Networks (“WLANs”) has recently experienced explosive growth as multiple applications and services now demand high throughput networks. One of the key features of high speed WLANs is the use of Multiple Input, Multiple Output (“MIMO”) antenna technology that offers significant increases in data throughput and link range without additional bandwidth or transmit power. Performance improvements are also achieved with the use of Orthogonal Frequency-Division Multiplexing (“OFDM”) modulation to convert a wideband channel into multiple narrowband channels in order to avoid inter-symbol interference (“ISI”).
A MIMO-OFDM channel is described with fine granularity by Channel State Information (“CSI”), which represents the current conditions and properties of the channel. CSI is provided in the 802.11n hardware by analyzing received packets with training sequences in the packet headers. For network algorithms such as rate selection, access point (“AP”) association, channel assignment, etc., to make a timely, optimal decision, accurate CSI estimates under various settings (e.g., different number of spatial streams, transmission antennas used, transmission powers, etc.) must be known. However, some of these settings might not be sampled in recently received packets and additional packet transmissions are required to obtain the complete CSI to accurately characterize the channel. This extra process consumes bandwidth and increases latency, and hence such unnecessary sampling should be avoided.